The present invention relates to an image forming apparatus using such electro-photographic process as laser printing, image copying etc., particularly to an image forming apparatus using plural colors of toner.
The electro-photographic process executes process of electrifying a photosensitive medium, exposing an image on the medium and developing the exposed image for each color of an image and superimposes images of respective color the exposed image. In the electro-photographic process, a multi-color image is usually formed by repeating a single color photographic process.
In Japanese Patent Application Laid-Open No. 95471/1988, in order to keep the concentration of each color image constant, it is presented that the reflectance of an image developed on an image formed on an image forming base medium is measured and fed back to image forming control conditions. Then, the accurate and stable measurement of toner deposit amounts is important for the above-mentioned method in which each color toner deposit amount is measured and the image forming control conditions are adjusted based on the measured toner deposit amounts. Since color toner of yellow toner, magenta toner and cyanine blue toner used for forming a color image absorbs a part of light wave band peculiar to each color, a decrease ratio of light reflected from the color toner due to increase of the toner deposit amount is smaller than that of black toner. Therefore, it is generally considered that the accurate measurement of color toner deposit amounts is difficult. A method of accurately measuring the deposit amounts of color toner and black toner is described in Japanese Patent Application Laid-Open 280869/1987. In FIG. 15, a relation between output changes of a photo-sensor and toner deposit amount changes, measured by a photo-sensor arranged at a position for measuring regular reflection as shown in FIG. 14, is shown. From FIG. 15, it is apparent that the output changes of the photo-sensor to the deposit amounts of color toner are smaller than those of black color. And, in the above-mentioned patent application, it is shown that such results as shown by FIG. 17 are obtained by measuring light reflection with a photo-sensor arranged as shown in FIG. 16, for receiving irregularly reflected light. Then, in the application, it is devised that a photo-sensor arranged at the position as shown in FIG. 14 is used for measurement to black toner, and a photo-sensor as shown in FIG. 14 is used for measurement to color toner. In Japanese Patent Application Laid-Open 245177/1986, in order to measure the toner deposit amount more accurately, it is disclosed that a standard high concentration image forming part and a standard low concentration image forming part are provided on a part of a photosensitive medium, respectively, the measurement being carried out by forming a color image on the former part and a black image of the latter part.
In the above-mentioned conventional technique of measuring increase of irregular reflection components due to increase of the toner deposit amount, the technique has several problems in measuring color toner deposit amounts. The results by the technique is easily affected by toner depositing manners or surface states of deposited toner, and the measured values are likely to be uncertain since the technique measures the irregular reflection. The conventional techniques measure a small part of the whole light reflected from deposited toner, which makes the measured values uncertain.
The conventional technique discussed above measures the toner deposit amount by using a reflection mode transition of light from the regular reflection to the irregular reflection. The reflection mode transition also occurs in a reflection from black toner. In the conventional techniques, a photo-sensor for measuring the toner deposit amount is arranged at the position receiving the regular reflection. Light amount received by a photo-sensor rapidly decreases corresponding to increase of the toner deposit amount since the reflection mode transits from the regular reflection to the irregular reflection. The toner deposit amount for gaining a sufficient concentration of an image is about 1 mg/cm.sup.2. However, light amount received by a photo-sensor is very small at the toner deposit amount 1 mg/cm.sup.2 as shown in FIG. 15 in the measurement of black toner. Then, Accuracy and certainty of the measurement of black toner is also not sufficient.
In case an intermediate transfer medium is used, a toner concentration of a transfered image greatly changes due to such a change as environment variation of the intermediate transfer medium. So it is effective to measure the toner deposit amount of an image formed on the intermediate transfer medium.
The above-mentioned conventional technique adopting two photo-sensors for measuring black toner and color toner makes a composition of the sensors, sensor corrections and a measurement control method, of a toner deposit amount measuring device complicated.
Another toner deposit amount measuring device for measuring the toner deposit amount on a developing roll is disclosed in Japanese Patent Application laid-Open No. 83057/1985. A toner deposit amount sensor of the measuring device is arranged opposite to the developing roll during toner deposit amount measurement and moved by rotation to a position not opposite to the developing roll, being cleaned and corrected with a cleaning device and a correction member in the rotating motion so as not to be contaminated by toner. However, the above-mentioned toner deposit amount measuring device does not have a means for containing and sealing up the toner deposit amount sensor, and the toner deposit amount sensor is likely to be contaminated by scattered toner particles, which causes decrease of effective light amount emitted from a light-emitting element of the sensor and degrades receiving of light reflected by a pattern image for measuring the toner deposit amount. Furthermore, the measuring device does not have such a composition as prevents effects by the light from the outside and the scattered toner particles from depositing on the correcting member. Then, it is difficult accurately to measure a reflectance of a toner image during long period by the above-mentioned measured device.